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Measuring up to the Gold Standard

07.07.08

The fiber optic wing shape sensor team is testing its work, which will compare a fiber optic-based system with conventional sensors in flight. The sensors, located along a cable the thickness of a human hair, aren't visible in the center of the wing, but could have ramifications for all future aircraft and spacecraft. Team members include, clockwise from left, Anthony "Nino" Piazza, Allen Parker, William Ko and Lance Richards. NASA Photo / Tom Tschida.
The Ikhana uninhabited aircraft system is flying research missions with an advanced sensing technology installed on its wings that measures and displays the shape of the aircraft's wings in flight.

The new sensors, which incorporate fiber optic sensing technology, are located side by side with traditional sensors. Generations of aircraft and spacecraft could benefit from work with the new sensors if the sensors perform in the sky as they have in the laboratory, said Lance Richards, Dryden's Advanced Structures and Measurement group lead.

Aerospace companies, NASA mission directorates and other government entities are watching the experiments with interest. The weight reduction that fiber optic sensors would make possible offers dramatic possibilities for reducing costs and improving fuel efficiency, Richards said.

The potential for weight reduction, however, is but one small part of the picture. This technology also opens up new opportunities and applications that would not be possible with conventional technology. For example, the new sensors could enable adaptive wing-shape control - the concept of changing a wing's shape in flight to take advantage of aerodynamics and make the aircraft more efficient.

Six hair-like fibers located on the top surface of the Ikhana's wings provide about 2,000 strain measurements in real time. The fibers are so small that they have no significant effects on aerodynamic lift and weigh less than two pounds. The fiber optic sensors themselves are so small that they could eventually be embedded within composite wings in future aircraft, he added.

"The applications of this technology are mind-boggling," Richards said.

A Winning Proposal

Although the new sensors on the Ikhana, which are located on fibers that are the diameter of a human hair, are not visible, the sealant used to apply them can be seen in this view from above the wing. NASA Photo / Tony Landis.
Richards and his team submitted a plan to fly the fiber optic wing shape sensors on the Ikhana's long wings when proposals for research with the remotely piloted aircraft were requested in late 2006. NASA's Aeronautics Research Mission Directorate, through which the basic research development project with the sensors is funded, is supporting algorithm and systems development, instrument and ground test validation.

When using the fiber optic sensors, researchers do not require analytical models for determining the answers they seek for strain and other measurements on the aircraft because data derived with the sensors include all of the actual measurements being sought.

"There are 3,000 sensors on Ikhana that are imperceptibly small because they're located on fibers approximately the diameter of a human hair," Richards explained. "You can get the information you need from the thousands of sensors on a few fibers without the weight and complexity of conventional sensors. Strain gauges, for example, require three copper lead wires for every sensor."
The research team is taking that concept a step further by comparing results obtained with the fiber optic wing shape sensor against those of traditional sensors to validate the new sensors' accuracy.

"There are 16 strain gauges on the wing that are co-located with the new sensors for side-by-side comparison," Richards said.

Proving the Technology

Larry Hudson and James Smith work on a ground validation test with the new sensors that led to validation flights with the aircraft. NASA Photo / Tony Landis.
The Ikhana flights will represent one of the first comprehensive validations of fiber optic sensor technology in flight, an enabling step toward using the fiber optic sensors for active wing shape control. Richards said the team is pursuing flying the fiber optic wing shape sensors for research into aeroelastic wing shape control on Dryden's F/A-18 no. 853, which was used as the Active Aeroelastic Wing project testbed, if tests on the Ikhana go as planned.

"Active wing shape control represents the gleam in the eye of every aerodynamicist," he said. "If the shape of the wing can be changed in flight, then the efficiency and performance of the aircraft can be improved throughout the aircraft's widely disparate flight regimes, from takeoff and landing to cruising and maneuvering."

Another benefit of the lightweight nature of fiber optic sensors is that thousands of sensors can be left on the aircraft during its lifetime, gathering data on structural performance, he said. At the heart of the technology is the comprehensive measurement of strain, the parameter that allows engineers to determine the stress experienced by aircraft structures.

For the first time, Richards explained, aircraft designers can use the sensors to gain important information regarding the efficiencies of their designs and the validity of their assumptions. By knowing the stress levels at thousands of locations on the aircraft, designers can more optimally design structures and reduce weight while maintaining safety. The net result could be reduction in fuel costs and an increase in the distance aircraft are capable of traveling.
There are other potential safety applications for the technology. If an aircraft structure can be monitored with sensors and a computer can manipulate flight control surfaces to compensate for stresses on the wings, structural control can be established to prevent situations that might otherwise result in a crash. Richards said the fiber optic strain and wing shape sensing also has the potential to revolutionize ground-testing methods, analysis and flight research.
By extension, it could revolutionize wing design efficiency and that of wind turbines by making the wing more efficient by a few percent.

"A few-percent improvement equals a huge economic benefit," Richards said. "The sensors could also be used to look at the stress of structures, like bridges and dams, and possibilities extend to potential biomedical uses as well."

Origins

Richards, who holds a doctorate in mechanical engineering, watched with interest as fiber optics began to evolve in research laboratories throughout the 1980s. In the mid-1990s, he began to consider aerospace applications of that technology for sensors. An internal Dryden Code R competition for grants resulted in seed money enabling him and his small team to begin filling in gaps in research that would validate sensing technology for broad use. No commercial technology is available for real-time wing shape sensing, he added.

Once he obtained funding, Richards assembled a team of Dryden engineers. He and William Ko developed optimized structural algorithms, Allen Parker developed the systems and originated data processing algorithms, and Anthony "Nino" Piazza is the team's strain measurement expert. The group developed the sensor and the supporting avionics system and algorithms that make the fiber optic wing shape sensors work, and has applied for two patents for use of the algorithms developed during the course of the work.

In 1996 Richards developed a partnership with a NASA Langley Research Center team led by Leland Melvin, currently an astronaut and a former Langley engineer. Increasingly complex research examined a technique patented by Mark Froggatt, formerly of Langley, for applying fiber optic technology to flight. Gains Melvin's team made in 1994 provided a foundation for its 1996 work in the development of a facility that provides a means of performing advanced sensor and laser research for development of military, space and civil aircraft health-monitoring systems.

Richards' team began incrementally, through laboratory experiments, to accumulate research showing that fiber optic sensors could match performance by the gold standard in sensors, strain gauges. A baseline was developed and the fiber optic sensors performed within a few percentage points of strain-gauge performance in side-by-side laboratory comparisons. The accuracy of the fiber optic sensors was proven - important data he felt were missing in the literature prior to his own research, he said.

After a lot of perseverance, the crucible of flight research is about to distill the scientists' contentions. Fiber optic wing shape sensors will compete head-to-head with strain gauges in the rigorous environment of flight where vibration, temperature and other challenges will prove the sensors' merit.